1. Field of the Invention
The present invention relates to a thermoelectric semiconductor material, a thermoelectric element, a method of manufacturing these and a method of manufacturing a thermoelectric module and a device for manufacturing a thermoelectric semiconductor material, and, in particular, relates to a material, a method of manufacture, and a manufacturing device that effectively contribute to improvement of thermoelectric performance.
2. Description of the Related Art
Thermoelectric elements making use of the thermoelectric phenomenon have conventionally been utilized in heat exchangers and/or temperature sensors. The thermoelectric phenomenon is a general term for the Peltier effect, Thomson effect and Seebeck effect. These will be described as follows.
The Peltier effect is the phenomenon whereby, when current flows to a junction of different metals, heat is generated or absorbed at this junction; the Thomson effect is the phenomenon whereby when current is passed to metal having a temperature gradient, generation or absorption of heat occurs within this metal. A Peltier element which is used as an electronic cooler is a thermoelectric element utilizing the above Peltier effect.
The Seebeck effect is a phenomenon whereby an electromagnetic force is generated at the high-temperature side and low-temperature side of a sample when a junction of different metals is maintained at different temperatures; thermocouples that are employed as temperature sensors are thermoelectric elements utilizing this Seebeck effect. Since such thermoelectric elements are easy to handle and have a simple construction and stable characteristics, research and development is proceeding in many places aimed at applying these to temperature regulation of semiconductor lasers and small freezers.
As the material for forming such thermoelectric elements, alloys are currently employed comprising one or two selected from the group consisting of bismuth (Bi) and antimony (Sb) and one or more selected from the group consisting of tellurium (Te) and selenium (Se). These compounds are laminar structure compounds and constitute semiconductor materials having anisotropy in their thermoelectric characteristics produced by their crystal structure.
Various techniques such as unidirectional solidification, hot pressing, or extrusion are known in order to process the semiconductor material consisting of such a laminar structure compound in order to increase fineness of the crystal grains and the degree of their alignment.
Unidirectional solidification is a method of forming an ingot in which the direction of crystal growth is controlled; by this method, polycrystalline material of excellent alignment is obtained. As a specific example of the uniaxial solidification method, the Bridgeman method is known. However, polycrystalline materials produced by such uniaxial solidification methods are subject to the problem of poor material strength. The polycrystalline materials obtained by this method are therefore undesirable for use as thermoelectric semiconductor elements without modification.
Hot pressing is a method of producing polycrystalline material wherein improvement in the material strength is sought to be achieved by uniaxial compression of powder etc. of an ingot. The reason for applying uniaxial pressure is to forcibly align the crystal orientations by external pressure. By means of such methods, the problem referred to above of the material strength of the uniaxial method being weak is solved, and polycrystalline material of excellent alignment is obtained.
The extrusion method is a method wherein powder or material formed of this powder is introduced into a die and pressure molding is performed whilst extruding the material in this die using a punch. Prior art references disclosing this method of extrusion include Japanese patent application laid-open No. 138789/1988, Japanese patent application laid-open No. 186299/1996, and Japanese patent application laid-open No. 56210/1998. By means of this method, since a strong force is applied to the material as a whole, finer crystal grains can be obtained and material strength is also improved.
Consequently, hot pressing, cold pressing, and the extrusion method are currently widely employed as methods of manufacturing thermoelectric semiconductor elements, for reasons of alignment of the crystals and material strength.
However, in recent years, thermoelectric elements having even better thermoelectric properties are being demanded, and a novel technique in which the prior art described above is further developed is therefore sought.
A first object of the present invention is therefore to provide a thermoelectric semiconductor material or method of manufacturing an element and method of manufacturing a thermoelectric module which should be effective for improving thermoelectric performance.
Also, a thermoelectric element (thermoelectric module) employed in electronic cooling and thermoelectric power generation, as shown in FIG. 29, is constituted by forming PN element pairs by joining P-type semiconductors 110 and N-type semiconductors 120 through metallic electrodes 130, and arranging a plurality of these PN element pairs in straight rows; depending on the direction of the current flowing through the junction, heat is generated at one end while the other is cooled. For the material of such thermoelectric elements, materials are employed of large figure of merit Z (=.alpha..sup.2 /.rho..kappa.) expressed by the Seebeck coefficient .alpha., resistivity .rho. and thermal conductivity .kappa., which are specific characteristics of the substance, in the temperature range of its utilization.
Most thermoelectric semiconductor materials possess anisotropy of thermoelectric performance due to their crystalline structure. Specifically, the figure of merit Z is different depending on crystal orientation. Single-crystal materials are therefore employed by passing current in a crystal orientation which gives large thermoelectric performance. In general, anisotropic crystals possess a tendency to cleavage and are of low material strength, so, as members for practical use, rather than single-crystal materials, polycrystalline materials wherein the crystal orientations are aligned to give large thermoelectric performance by uniaxial solidification using the Bridgeman method etc.
However, polycrystalline materials are still brittle in material strength albeit not to the degree of single-crystals, and so suffer from the problems of cracking or chipping of the elements during element processing.
Specifically, a polycrystalline material that is typically used for thermoelectric cooling elements is Bi.sub.2 Te.sub.3 based thermoelectric material, which consists of a mixed crystal system of bismuth telluride (Bi.sub.2 Te.sub.3), antimony telluride (Sb.sub.2 Te.sub.3), and bismuth selenide (Bi.sub.2 Se.sub.3). This Bi.sub.2 Te.sub.3 based thermoelectric material is of hexagonal crystal structure, having a structure in which layers of Bi and layers of Te are laminated perpendicular to the hexagonal crystal C axis. Due to this crystalline structure, it possesses electrical and thermal isotropy, the thermoelectric performance being better in the direction of the C plane than in the direction of the C axis. Thermoelectric elements are therefore employed produced by ingots in which the direction of crystal growth is controlled to be the orientation which gives best thermoelectric performance, by a uniaxial solidification method. However, since, in regions where adjacent Te layers are laminated in the crystalline structure, the Te atoms are mutually coupled by Van der Waals forces, they are subject to severe cleavage. There were therefore the problems that yield was very poor due to occurrence of cracking or chipping in the-cutting step etc. to obtain the thermoelectric elements from this brittle crystalline material and that the thermoelectric elements (thermoelectric modules) did not possess durability.
There have therefore previously been attempts to obtain elements of improved material strength by sintering material obtained by crushing and pulverizing ingots (solidified material).
An enormous improvement in material strength is obtained as sintered materials have in fact no tendency to cleavage when compared with ingots, but, as compensation for this material strength, the alignment of crystal orientation is random or crystal orientation has only a gently sloping distribution so the degree of alignment is low, with the problem that thermoelectric performance (figure of merit Z) is inferior to that of ingots.
Thus, no thermoelectric semiconductor material previously existed possessing both fully satisfactory strength and thermoelectric performance. Accordingly, the present applicants have already applied for patents (Japanese patent application No. 2110624/1997, Japanese patent application No. 269389/1997) for inventions whereby thermoelectric semiconductor materials combining both fully satisfactory strength and thermoelectric performance are produced by undergoing a hot upset forging processing step, which is a kind of plastic deformation processing. Specifically, both strength and thermoelectric performance are improved compared with the conventional ingots or sintered materials by lining up the directions of the C planes by compressive external force by hot (upset) forging of a sintered body.
In recent years however, thermoelectric elements are being demanded having a finer structure with even better strength and close crystal alignment without necessarily being single crystals. Specifically, elements are being demanded wherein:
1) By conferring higher strength on the thermoelectric element, chipping and cracking are reduced and manufacturing yield is raised; PA1 2) The directions of the crystals of the thermoelectric element are brought closer to a single direction and anisotropy of the thermoelectric performance is raised, thereby improving thermoelectric performance; and PA1 3) By making the crystal grains even finer, the thermal conductivity .kappa. is lowered, so improving thermoelectric performance (the figure of merit Z becomes larger as the thermal conductivity .kappa. becomes smaller). PA1 where Z=figure of merit.times.10.sup.-3 (1/K); a=Seebeck coefficient (.mu.V/K); .alpha.=conductivity (.mu..OMEGA..sup.-1.multidot.cm.sup.-1); .kappa.=thermal conductivity (mW/cmK); and .rho.=electrical resistivity (.mu..OMEGA..multidot.cm).
It should be noted that it is said that when the fineness of the crystalline structure is increased, the thermal conductivity .kappa. falls.
Accordingly, as a method of processing having the capability to satisfy these demands, extrusion molding processing, which is a type of plastic deformation processing, may be considered. FIG. 30 shows a conceptual diagram of a prior art extrusion molding process. Inventions in which a thermoelectric element is manufactured by extrusion molding processing are disclosed in Japanese patent application laid-open No. 138789/1988, Japanese patent application laid-open No. 186299/1996, and Japanese patent application laid-open No. 56210/1998.
As shown in FIG. 30 in the extrusion processing described in these publications, by extruding from a cylindrical extrusion port 140a of a die (extrusion mold) 140 by applying a pressing force by means of a punch in the extrusion direction D on to a cylindrical sintered body 150 of thermoelectric semiconductor material, a cylindrical extrusion molding 150' of smaller diameter than before molding is formed.
In the extrusion molding process, plastic deformation is performed whilst subjecting the cylindrical sintered body 150 to external force from the entire peripheral direction L of the circumference in extrusion mold 140 (whilst compressing uniformly in the circumferential direction). As a result, the external force acting on the material within the metal mold is greater than in the case of hot forging, and force can more easily be applied to the entire material.
Consequently, destruction of crystals due to plastic deformation and dynamic recrystallization during molding occur very satisfactorily, resulting in increased fineness of the crystal grains compared with hot forging. Due to the increased fineness of the crystal grains, the thermal conductivity .kappa. is lowered and thermoelectric performance is improved.
Also, since the degree to which external force is applied is better than in the case of hot forging, it is easier to align the C faces, increasing anisotropy and enabling thermoelectric performance to be improved.
Also, whereas, in the case of hot forging, the state of deformation is different at all the locations within the molding, resulting in scatter of the distribution of the thermoelectric characteristic, in the case of extrusion molding, there is little scatter of the distribution of the thermoelectric characteristic within the molding. Concomitantly with this reduction in scatter of the thermoelectric characteristic, the strength of the material is improved, thereby enabling the yield during manufacture to be increased.
By such extrusion processing, the above demands 1) to 3) can be met.
However, the conventional extrusion molding processing involves forming a cylindrical extrusion molding 150'. When a cylindrical extrusion molding 150' is formed, the following problems arise, which made it impossible to adopt the conventional extrusion molding process.
Specifically, with the conventional method, it is a presumption that the cylindrical extrusion molding 150' obtained by extrusion molding is further cut up into disc shapes, and the thermoelectric module is assembled from the disc-shaped thermoelectric elements obtained.
However, although of course the disc-shaped thermoelectric elements described above are sometimes used, in recent years thermoelectric elements cut into rectangular prism shape are mostly used for the thermoelectric modules that are manufactured. Since when rectangular solid thermoelectric elements are cut from a cylindrical extrusion molding 650' the remaining portions that are cut away from the rectangular prisms must be discarded yield is greatly lowered.
Also, during extrusion molding, impurities may adhere to the side face of the cylindrical extrusion molding 150', minute cracks may be formed therein, or lubricant may adhere thereto. These are also factors that increase the resistance and lead to a lowering in thermoelectric performance. They are also factors that cause a drop in strength.
It is therefore necessary to remove the cracks etc. or impurities adhering to the surface of cylindrical extrusion molding 150', and to clean it, in order to raise its strength and improve thermoelectric performance.
However, grinding by mechanical processing such as cutting of cylindrical material 90' involves considerable problems. Specifically, there is a risk in the number of steps being, multiplied, as, since cylindrical material 150', is used, the material must be mounted on a lathe and cut whilst the material is rotated so as to grind its surface. Furthermore, although strength is said to be improved, the material is still brittle, so when cutting processing is performed using a lathe there is a risk of the material being destroyed.
In practice therefore it was impossible to grind the surface of cylindrical extrusion molding 150' by mechanical processing.
Also, as described above, in the surface of an extrusion molding 150' obtained by conventional extrusion processing, minute cracks were formed. In order to eliminate the cracks, a step of raising the density of extrusion molding 150' is necessary. Specifically, a step of consolidating after the step of extrusion may be considered. A consolidation step is a step in which density is increased by compressing the extrusion molding by sealed forging by inserting the extrusion molding into a metal mold.
However, consolidating cylindrical extrusion molding 150' without disturbing the ordered alignment obtained by the extrusion is impossible with current technology.
Thus, if extrusion molding 150' is of cylindrical shape, there are the problems that the yield on cutting out rectangular solid thermoelectric elements is low, a surface grinding step is impossible in practice, and a consolidation step is also impossible in practice; it was therefore not possible to adopt the conventional extrusion molding processing method.
The present invention was made in view of the above and a second object thereof is to raise the yield of the step of cutting out rectangular solid thermoelectric elements and to make possible a surface grinding step and a consolidation step.
Incidentally, in the conventional extrusion molding processing method, an extrusion molding 150' of cylindrical shape of smaller diameter can be formed by drawing with uniform compression in the entire circumferential direction L of the circumference of a cylindrical sintered body 150.
The thermoelectric performance of a thermoelectric element is a maximum when the electric current or heat current flows in a direction in which the C planes at the basal face of the crystal (hexagonal crystalline structure) are lined up (horizontal direction to C planes) If the thermoelectric performance of the thermoelectric element is raised, the maximum temperature difference can be made large, enabling a thermoelectric module of good cooling to be efficiency obtained.
Lining up and aligning the C faces of the crystals constituting the structure in a specific direction during extrusion molding is therefore important.
However, as described above, the present inventors discovered that if the method of drawing while compressing uniformly a cylindrical sintered body 150 in the entire circumferential direction L of its circumference is adopted, it is difficult to get the C faces lined up in a specific direction.
Furthermore, if the method of drawing while uniformly compressing cylindrical sintered body 150 in the entire circumferential direction L of its circumference is adopted, impurities adhere to the surface of the side faces of cylindrical extrusion molding 150', fine cracks are formed, and lubricant can easily adhere thereto. These are factors that tend to cause increase in resistance and cause lowering of thermoelectric performance. They are also factors that cause lowered strength.
The present invention was made in view of the above circumstances and, in addition to solving the second object mentioned above, has a third object making it easy to line up the C faces in a specific direction and enabling cracks etc. of the extrusion molding surface to be reduced.
Incidentally, in the prior art extrusion molding processing method, a method may be adopted of forming an extrusion molding 150' by sintering the powder that constitutes the raw material of the thermoelectric semiconductor material, introducing this sintered body 150 into an extrusion mold 140, and extruding this to form an extrusion molding 150'.
In this case, the sintering step and extrusion step are as shown in FIG. 31.
Now, in the sintering step as shown in FIG. 31(a), the thermoelectric semiconductor material is inserted into a cylindrical mold and the material is compressed by applying pressure from the cylinder head in the direction of arrow B. Heating is simultaneously conducted so that the material is sintered to form a sintered body 150. In this process, C faces, which are the basal face of the crystals of hexagonal structure, are aligned in the perpendicular direction Cx with respect to the direction of compression B.
Next, in the extrusion step as shown in FIG. 31(b), pressurizing force is applied in the direction of arrow D to the cylinder head of cylindrical sintered body 150, causing sintered body 150 to undergo plastic deformation within extrusion mold 140 by being subjected to external force from its entire circumferential direction L of its circumference. As a result, extrusion molding 150' is formed. The basal faces constituting the C faces of the hexagonal crystal structure are then aligned in the horizontal direction Cx with respect to the extrusion direction D.
Consequently, the direction of lining up of the C faces differs by 90.degree. as between the sintering step and the extrusion step and this makes lining up of the C faces difficult to achieve. As a result, even if current flows in the same direction as the extrusion direction with respect to extrusion molding 150', the demanded thermoelectric performance is not obtained.
The present invention was made in view of the above circumstances. In addition to the above second object and third object, it has a fourth object of improving thermoelectric performance by achieving coincidence of the direction of current flow and the direction in which the C faces are most strongly aligned by improving the degree of alignment of the C faces, by making the direction of pressure application in the pressurization step (sintering step) prior to the extrusion step coincide with the direction of pressure application during the extrusion step.